Holography makes it possible to record and subsequently reestablish the amplitude and phase distributions of a wavefront. In this case, an interference pattern of coherent light reflected from an object and light coming directly from a light source is recorded on a recording medium, e.g. a photographic plate. If the interference pattern, also referred to as a hologram, is illuminated with coherent light, a three-dimensional scene arises spatially. In order to generate the hologram by means of known methods or techniques, a real three-dimensional object is usually used, the hologram then being referred to as a genuine hologram. However, the hologram can also be a computer-generated hologram (CGH).
As reversible recording media for CGHs, use is made of light modulators, such as, for example, LCD (Liquid Crystal Display), LCoS (Liquid Crystal on Silicon), EASLM (Electrically Addressed Spatial Light Modulator), OASLM (Optically Addressed Spatial Light Modulator), which modulate the phase and/or the amplitude of incident light.
Electrically addressable spatial light modulators (EASLM) are very often used in reproduction devices or displays. In this case, an EASLM can be defined as a spatial light modulator which is constructed from discrete elements which are connected to an electrical circuit and are likewise driven via the latter. However, EASLMs for use in holographic reproduction devices for three-dimensional representation have considerable disadvantages, such as for example the limited number of modulation elements, also called pixels, the small filling factor and the relatively low resolution resulting therefrom.
In order that, however, a large three-dimensional scene can be offered, or a large observer region made possible for the observer, the EASLM must have a large number of modulation elements or pixels which are arranged very close together in order that a high filling factor can be achieved. In practice, however, this can only be achieved with high complexity and is associated with above average costs, with the result that good economic viability cannot be obtained.
Therefore, attempts have already been made to use optically addressable spatial light modulators (OASLM) for this purpose. An OASLM is a light modulator which can be used to generate an optically controllable change in the amplitude transparency and/or phase transparency. It has considerable advantages over an EASLM, particularly in the case of application in a reproduction device. The principal advantage resides in its analogue behaviour or in the fact that it is not pixelated. This means that there are no discrete pixels and therefore no filling factor and no sampling interval. Consequently the resolution of an OASLM is significantly higher than that of an EASLM. However, the problem resides in the addressing of an OASLM, that is to say in the recording of information or holograms thereon.
Various solutions have already been disclosed for recording a hologram on an optically addressable spatial light modulator. One solution is referred to as Active Tiling™ and is disclosed for example in U.S. Pat. No. 6,753,990 B1 or US 2004/0196524 A1. These documents describe for three-dimensional holographic representation the use of an electrically addressable spatial light modulator (EASLM) which is relatively small in terms of its size in conjunction with a relatively large optically addressable spatial light modulator (OASLM). In this case, holographic image data are displayed on the EASLM, said image data being sequentially focused by means of a microlens arrangement onto different regions or segments of the OASLM, and a hologram thus being written there. However, the OASLM is not written to directly and the hologram is not recorded directly, rather partial holograms are generated on the EASLM and transferred to the OASLM by means of the microlens arrangement.
However, new types of OASLM technologies, for example colour-doped OASLMs, expect a resolution of approximately 300 lp/mm to 1500 lp/mm and higher. With such a high resolution it is possible to generate holographically high-quality reconstructions in conjunction with large observer regions in comparison with the prior art to date. In order to use such an OASLM for the representation of three-dimensional scenes to be reconstructed, however it is necessary to write to the OASLM a hologram with correspondingly high resolution. It is necessary therefore for the OASLM to have regions or segments which are not larger than 3 μm, by way of example. However, the direct recording of a hologram on the OASLM is rather difficult with the aid of light sources, since the size of the light sources is generally larger than 10 μm and the total number of light sources required should not be too large. Furthermore, the recording of the hologram does not yield high-quality results with scanning systems or deflection systems, such as mirrors or prisms, in the case of a corresponding segment size of the OASLM, such that these solutions are likewise disadvantageous. Moreover, most of the systems existing hitherto can only be used for the current OASLM technology producing a resolution of 30 lp/mm to 100 lp/mm.